A hammer drill chuck can have a plurality of clamp jaws centered about and axially advanced against the bit, the clamp jaws defining a drill-bit cavity coaxial with the chuck axis for receiving the shank of the bit and are simultaneously moved centrally toward the chuck axis, and further having means for rotational locking of the bit against rotary slipping in the chuck.
A working tool with a smooth cylindrical shaft held slidably axially between clamp jaws in a chuck of the foregoing type is taught in German Patent document DE-OS 31 33 085. The catch device here is an adjustable V-block or dog braceable from the exterior against the shaft of the working tool. The chuck dog secures the bit against relative rotation and is axially slidable at least to the extent of the hammer stroke as well as radially according to the diameter of the working tool shaft.
The braced chuck dog takes part in the hammer motion of the drilling tool and hence its mass may damp the hammer motion or the hammer blows on the working tool. Above all, the bracing of the chuck dog on the cylindrical working shaft, which also serves to restrain the drill bit rotationally, should be sufficient to permit a series of hammer strokes to be applied. The chuck dog more or less digs into the drill bit shaft, but the grip of the chuck dog can be loosened. The chuck dog can then slip axially as well as in the circumferential direction which of course defeats the desired locking of the drill bit.
A percussion drill bit with at least one axially closed axially extending groove on its shank is described in German utility model DE-GM 81 32 988, in which the lengthwise groove is divided by at least one cross-piece in conformity to the contour of the shank into a plurality of sections. This structure of the shank makes possible the use and interchangeability of drill bits both in the common drilling machine and also in the hammer drill. The cross pieces offer the possibility, being positioned laterally across the axially extending groove, for the clamp jaws of the chuck to grasp the drill bit in the case of the hammer drill. On the other hand the groove structure makes possible holding the drill bit in specially constructed percussion drilling device. A number of these grooves exist in the drill bit shaft of this prior art design and they are engaged by locking elements of the drill bit holder so as to be secured rotationally, but with some freedom of limited axial play. However, there is no possibility in this prior art drill chuck, having the locking elements for the grooves of the drill bit, to clamp a drill bit which has a smooth, cylindrical surface without such grooves.